Sensors for reducing gas molecules

ABSTRACT

Provided is a gas molecule sensor, characterized in that the sensing element is a polycrystalline tin oxide film having a thickness less than 1 μm. The sensing element is produced by electrolytic deposit of a tin film on an insulating support in an electromechanical cell, where the anode is comprised of tin and the cathode is a conductive film applied on the surface of the insulating support at one of its ends, the two electrodes being separated by an electrolyte comprised of a tin salt solution, and by passing a constant current through said cell. The deposit step is followed by an oxidizing step.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a sensor for reducing gas molecules, thesensitive element of which is tin oxide.

2. Description of the Related Art

The detection and measurement of the concentration of harmful moleculesin an atmosphere constitutes a very major challenge, especially becauseof the increase in emissions of industrial or municipal origin. It hasbecome necessary to detect the presence of harmful molecules in everdecreasing concentrations, below the concentrations of danger to humans.The case of carbon monoxide CO, found in particular in exhaust gases andin cigarette smoke, is one example, this gas being fatal at extremelylow concentrations, of the order of 1 ppm. Furthermore, these samemolecules must be detected in certain industrial or technologicaldevices such as fuel cells, in which very low concentrations cause, forexample, catalysts to be poisoned. The sensitivity threshold and thespeed of detection are the two important parameters of a sensor of thistype.

It is known to use tin oxide sensors to detect reducing gas molecules.The conductivity of tin oxide, which is an intrinsic semiconductor,varies according to the content of reducing molecules in the atmosphere.It is accepted that, in a nonreducing atmosphere, oxygen is adsorbed atthe grain boundaries and partially repels the grain boundary electronicstates, making the material highly resistive. In the presence ofreducing molecules, the oxygen concentration at the grain boundariesdecreases, thus releasing the electronic states near the surface of thegrains, on either side of the actual contact surface. The existence ofthese electronic states allows conduction of electrons across the grainboundaries. It is thus apparent that the sensitivity of the sensorshaving tin oxide as sensitive element increases when the oxide grainsize decreases, that is to say when the specific surface area increasesand when the quantum effect of repelling the electronic states towardthe interior of the grain is more effective. However, reducing the grainsize in general causes a reduction in the physical connectivity betweenthe grains, to the detriment of the conductivity.

Such sensors whose sensitive element is a tin oxide are described, forexample in the special issue “Gas-sensing Materials” of MaterialsResearch Society Bulletin, published in June 1998. They comprise apolycrystalline tin oxide layer on a support. In commercialized devices,tin oxide grain size is in general of the order of 1 μm. Also known aresensors containing polycrystalline tin oxide as active material in theform of grains having a size of less than 20 nm. These are essentiallylaboratory devices, produced by preparing the oxide from a metalobtained by sputtering, by laser ablation, by sol-gel processes, byprecipitation or by other chemical synthesis processes. When the oxideis obtained by a sol-gel process or by simple precipitation, it containsa solvent that has to be removed. In most cases, the oxide obtained isin the form of a powder of separate grains, which then has to besintered and fired so that it can be used for a sensor, and the physicalconnectivity of the grains is not guaranteed. Furthermore, sinteringresults in a three-dimensional material in the form of wafers thatcontain a large number of grains through the thickness. The responsetime of a sensor containing such an oxide will be longer the thicker thewafers, because of the time needed for the gas molecules to be detectedto diffuse into the core of the active material of the sensor. Anotherdrawback with sintering lies in the fact that it does not allow a sensorto be formed. The powders may be formed by processes consisting inincorporating the grains in an ink and then tracing the sensor using thetechniques used in ink-jet printers. However, this technique gives filmsin which the grains are not connected on a macroscopic scale in relationto their size and it introduces solvents into the grains.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a process forproducing the sensitive element of a sensor for reducing molecules,comprising a tin oxide layer consisting of very fine grains having verygood physical cohesion. This is why the subject of the present inventionis a process for producing the sensitive element of such a sensor, andthe sensor obtained.

The process according to the invention for producing the sensitiveelement of a sensor for reducing gas molecules is characterized in that:

-   -   during a first step, a layer of tin having a thickness of less        than 1 μm, preferably less than 400 nm, is deposited        electrochemically on the surface of an insulating support, by        placing said insulating support in an electrochemical cell        comprising an anode made of tin and a cathode which is a        conductive film applied to the surface of the insulating support        at one of its ends, the two electrodes being separated by an        electrolyte consisting of a solution of a tin salt, and by        passing a current of constant intensity through said cell;    -   during a second step, the tin layer obtained undergoes        oxidation.

The intensity of the current is such that it creates a current densityof between 1 μA/cm² and 5 mA/cm² in the plane perpendicular to theelectrodes passing through the growth front of the film which is beingdeposited.

The concentration of the tin salt in the electrolyte is preferablybetween 10⁻³ mol/l and 10⁻¹ mol/l.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one embodiment, the insulating support on which the layer has to bedeposited constitutes one of the walls of the electrochemical cell. Aninsulating plate approximately of the same dimensions as said insulatingsupport is parallel to said support and serves simply to close the cell,so as to protect it from air. The gap between the support and theinsulating plate defines the electrolyte volume. The support and theplate may be kept apart by the metal contacts which allow the current toflow between the tin anode mounted inside the cell and the cathodeformed by a conductive film deposited on that face of the support whichis turned toward the interior of the cell at one of the ends of saidface. The cathode may be a thin film of gold or of another metal. Forexample, if the metal forming the cathode is gold, a film of about 1000Å is appropriate. To bring the cathode formed by a gold film intocontact with the galvanostat that delivers the required current, it ispossible to use metal blades or wires. In a flat cell as defined above,the thickness e of the tin layer obtained is determined simply by theformula e=P×h×C/C_(M), in which h represents the distance between thetwo plates, that is to say the height of the electrolyte, C is thecation concentration of the electrolyte and C_(M) is the molarconcentration of the tin, that is to say the number of moles per literof tin in the solid state. P is a parameter that can be related to themobility of the cation and of the anion of the salt according to theformula P=1+(μ_(c)/μ_(a),) μ_(c) and μ_(a) being the cation mobility andthe anion mobility respectively. As a general rule, the cations andanions of a salt have very similar mobilities, and P is about 2. Thesimplified formula for determining e can therefore be written as:e=2h×C/C_(M).

At the start of the electrolysis step, the cathode serves to initiatethe formation of the tin, which forms as a layer on the support. The tinlayer deposited then remains adherent to the support.

The electrolyte may be an aqueous or nonaqueous solution of a tin salt.Aqueous solutions are preferred. The tin salt is preferably tin chlorideor tin sulfate. The salt concentration in the electrolyte may vary verywidely, depending on the quality and the compactness of the film that itis desired to obtain. A concentration of between 0.005 and 0.05 mol/l,more especially between 0.02 and 0.04 mol/l, is particularly preferred.It is preferred to use a tin salt solution kept away from air, so as toavoid the presence of hydroxides and carbonates.

Because the cell is operated at constant current, the growth rate of thetin layer is proportional to the intensity of the current, that is tosay to the flux of atoms.

When it is desired to obtain a uniform tin layer, the electrochemicalcell is subjected to a current having an intensity such that it createsa current density of between 0.05 and 5 mA/cm² in the planeperpendicular to the electrodes and passing through the growth front ofthe layer which is being deposited. In this case, when the electrolyteis a tin chloride solution prepared from a commercial tin chloridepowder, layers consisting of grains having a mean size of around 300 nmare obtained. When an electrolyte is used that is obtained by diluting,to 0.01 mol/l, just before use, a very concentrated and highly acidiccommercial tin chloride solution (for example solutions sold forsensitizing surfaces before silver plating in processes calledelectroless processes), the layer which is deposited on the supportconsists of disk-shaped wafers having a thickness of about 5 nm and acircumference of about 50 nm when the current density in theelectrochemical cell is less than 1 mA/cm², this structure beingdetermined by AFM (atomic force microscopy). Since the sensitivity toreducing atmospheres is dependent on screening phenomena between theadsorbed molecules and the active material of the sensor, it is thesmallest dimension of the grains which determines the properties of thesensor. In the present case, the performance is therefore particularlygood.

As a general rule, when it is desired to obtain a dendritic layer, theelectrochemical cell is subjected to a current intensity of between 1and 100 μA, preferably between 5 and 50 μA. At currents below about 1μA, macrocrystals form which are of little interest for sensors. Atcurrents above 100 μA, the metal is no longer deposited in the form of alayer on the support, but forms in the liquid. Electrolysis of the watermay also occur instead of metal deposition. When the electrolyte is ahighly acidic tin salt solution, it is necessary to impose a currentdensity of greater than 20 μA to obtain a dendritic layer. In oneparticular embodiment, a support on one of the faces of whichmicroscratches have been etched is used as support for depositing adendritic tin layer. In this case, the dendrites of the layer whichforms during the electrolysis follow the line of the microscratches. Apolycrystalline microwire is thus obtained, the thickness and the crosssection of which contain only a very small number of grains, or even asingle grain, through the thickness. These grains have a size of lessthan or equal to 1 μm, generally around 100 to 300 nm.

The tin layer deposited on the support may be oxidized by air oxidation.It is advantageous to carry out a simultaneous heat treatment in orderto accelerate the oxidation and the adsorption of oxygen at the grainboundaries. Oxidation and oxygen adsorption result in a significantincrease in the resistance of the layer up to a plateau which depends onthe thickness of the layer. When the layer is then subjected a reducingatmosphere, a significant drop in the resistivity is observed with a fewseconds at room temperature. When the layer is returned to a normalatmosphere, the resistance resumes its initial value in few tens ofseconds at room temperature.

The oxidation of the tin layer obtained during the first step of theprocess may be carried out after having constructed the sensor. In thiscase, when the tin layer has reached the desired length on the supportduring the first step, the electrochemical cell is opened, for exampleby removing the insulating plate that had served to close the cell onits upper part. The layer remains well adhered to the insulating supporton which it was deposited. After having removed the electrolyte, twosilver wires having a diameter of a few tens of a millimeter are bondedto the tin layer at its ends, the said silver wires being intended toserve as electrodes for the sensor. The assembly formed by theinsulating support carrying the tin layer provided with silver wires isthen subjected to an oxidizing atmosphere, for example the ambientatmosphere, optionally with a heat treatment.

The process for producing the tin oxide layer is particularlyadvantageous insofar as it produces very pure tin oxide in the form of avery thin layer directly on the support that will form part of thesensor. Furthermore, it allows working at room temperature, in a liquidmedium, without the use of difficult techniques. By controlling theintensity of the current applied during the electrolysis, it is possibleto define the thickness and the morphology of the layer deposited(homogeneous layer or dendritic layer). Furthermore, the fact that thecurrent flows through the grains during deposition of the tin in metalform guarantees that the tin oxide grains obtained after oxidation ofthe tin layer will be physically connected. Surprisingly, the oxidationof the tin layer does not cause the grains to be disconnected to anextent which could modify the conductivity properties of the oxide layerused as sensitive element of a sensor.

In the processes of the prior art for preparing tin oxide, it isnecessary to apply particular treatments, for example sinteringtreatments, or severe heat treatments such as calcining operations, inorder to obtain a conductive material. Finally, because the sensitiveelement of the sensor of the present invention is produced in liquidmedium, it is perfectly well suited to operating in such a medium.

According to the present invention, a sensitive element of a sensor forreducing gas molecules is formed by an insulating support coated with atin oxide layer and is characterized in that the tin oxide layer ispolycrystalline and has a thickness of less than 1 μm, preferably lessthan 400 nm.

In one particular embodiment, the tin oxide layer is substantially asingle grain through the thickness, that is to say the grain size issubstantially equivalent to the thickness of the oxide layer.

A sensor for reducing gas molecules may be formed by a sensitive elementaccording to the invention connected to two electrodes, a controller andan ohmmeter or an impedance bridge.

In one particular embodiment, the tin oxide layer is a homogeneouslayer.

In another embodiment, the tin oxide layer has a dendritic morphology.The grain boundaries are very accessible to the atmosphere. Furthermore,the tree structure of the grains can be likened to a wire structure. Thepresence of small grains along the path of the current, or along thiswire, is sufficient to make the sensor very efficient. It has also beenfound that some of the grains have wasp waists which, withoutconstituting two grain boundaries, remain preferential sites fordetecting the change in conductivity associated with the presence ofreducing molecules. The wire structure may be accentuated by using asupport in which microscratches have been etched. This technique is easyand makes it possible to obtain alignments of grains of any shape, sincethe grains are deposited along the scratch, which may have a straightshape or any shape.

The present invention will be described in greater detail with referenceto the examples given below.

Example 1

Preparation of an Oxide Layer

For the first step of the process, namely for preparing the tin film, anelectrochemical cell was used consisting of a microscope slide (servingas support for the deposition) and a thin microscope slide, both havingsides of 1.8 cm, kept 0.1 mm apart by two rectangular foils cut from ametal tin foil (sold by Goodfellow), one of the foils ensuringconnection between the generator and the cathode, the other foilensuring connection between the generator and the anode. The electrolytewas a 0.1 mol/l acidic aqueous tin chloride solution. The cathodeconsisted of a gold film 1000 Å in thickness, applied by evaporation atone of the ends of the internal face of the slide serving as support,the rest of this internal face having been presensitized by theevaporation of a layer of gold having a thickness of 20 Å, which doesnot conduct a current. The intensity of the current I delivered by thegalvanostat was 30 μA.

The measurements taken showed that the grain sizes were typicallybetween 200 and 600 nm.

The tin layer was then oxidized by heating at 100° C. for 4 hours in achamber in which air was circulated.

Example 2

Detection Trials

A sensor comprising as sensitive element a dendritic layer having grainsof around 400 nm was used to detect the carbon dioxide contained incigarette smoke. The contacts between the support, formed by themicroscope slide and coated with the tin oxide layer, and an ohmmeterwere produced by the use of silver lacquer deposited on the layer and ofsilver wires.

Cigarette smoke was drawn into a glass chamber having a volume of 0.5 1,the chamber being kept sealed. When the temperature of the smoke reachedroom temperature, an opening was made in the chamber with a diameter ofabout 5 cm, and the sensor was advanced to within a few cm of theopening, and then moved away. The advancing and retreating operationswere repeated several times, the resistivity of the sensor beingmeasured continuously.

A response time of 2.5 seconds was found when the sensor was broughtinto contact with the smoke, whereas the tin oxide sensors of the priorart, called Taguchi sensors, have a response time of about 10 seconds to1 min, depending on the type. When the sensor of the invention is movedaway from the smoke, it recovers its normal resistivity in 30 seconds.

Comparative measurements were taken with a similar sensor, but nothaving a tin oxide film, the sensitive element being formed by themicroscope slide carrying the silver lacquer and connected to theohmmeter by silver wires. No change in the resistivity was observed whenthis comparative sensor was brought into contact with the smoke.

1. A process for producing the sensitive element of a sensor forreducing gas molecules, characterized in that: during a first step, alayer of tin having a thickness of less than 1 μm is depositedelectrochemically on the surface of an insulating support, by placingsaid insulating support in an electrochemical cell comprising an anodemade of tin and a cathode which is a conductive film applied to thesurface of the insulating support at one of its ends, the two electrodesbeing separated by an electrolyte consisting of a solution of a tinsalt, and by passing a current of constant intensity through said cell;during a second step, the tin layer obtained undergoes oxidation.
 2. Theprocess as claimed in claim 1, characterized in that a current intensityis applied such that it creates a current density of between 1 μA/cm²and 5 mA/cm² in the plane perpendicular to the electrodes passingthrough the growth front of the layer which is being deposited.
 3. Theprocess as claimed in claim 2, characterized in that the current densityis between 0.05 mA/cm² and 5 mA/cm².
 4. The process as claimed in claim3, characterized in that the current density is less than 1 mA/cm². 5.The process as claimed in claim 2, characterized in that the currentdensity is between 1 μA/cm² and 100 μA/cm².
 6. The process as claimed inclaim 1, characterized in that the concentration of the tin salt in theelectrolyte is between 10-3 mol/l and 10-1 mol/l.
 7. The process asclaimed in claim 6, characterized in that the concentration of the tinsalt in the electrolyte is between 0.005 mol/l and 0.05 mol/l.
 8. Theprocess as claimed in claim 1, characterized in that: the insulatingsupport on which the layer has to be deposited constitutes one of thewalls of the electrochemical cell; an insulating plate approximately ofthe same dimensions as said insulating support is parallel to saidsupport and serves to close the cell, the gap between the support andthe insulating plate defining the electrolyte volume; the support andthe plate are kept apart by metal contacts which allow the current toflow between the tin anode mounted inside the cell and the cathodeformed by a conductive film deposited on that face of the support whichis turned toward the interior of the cell and at one of the ends of saidface.
 9. The process as claimed in claim 1, characterized in that thecathode is a thin gold film having a thickness of about 1000 Å.